For the basics, don't forget things like a water bottle, emergency food supply, and some comfortable way to carry your gear (whatever works for you).

What nathancarter says on the flash is good advice too. There are diffusers you can find online but I've had the best luck just building one using craft foam and velcro. If you look at this photo: http://accessaccess.deviantart.com/#/d5k9w89 it's #6 in the photo, it folds flat for storage, you can get the necessary pieces for less than $10. or so at a Michaels.

Yeah, don't forget snacks and water. This year was my first AX and there's so many cosplayers to photograph that I ended up not eating lunch, so the snacks and water are very helpful.

Depending on your style, you don't even need a flash. Some photogs prefer natural light and do amazing work with just their camera. But if you do bring a flash, Access's suggestion of making your own diffuser/bounce is easy and cheap and customizable. I'm lazy so I just buy them lol. For hall shots, the ceiling at LA convention center is really high and you're not usually near walls so there's not much to naturally bounce off of unless you can pull the cosplayer off to the side.

Me personally, walking around hallway shots = I bring just my camera + one flash with a small sling bag holding the batteries/memory cards/food/water. For shoots, I bring couple light stands and either leave them bare or use umbrellas/softboxes depending on what I'm doing.

If your style is straight-forward (no multi-flash, softboxes, etc), then a flash bracket is very handy at keeping the shadows behind the subject. I used one that rotated the camera so my flash always sat proper regardless of my camera orientation. If you are going to bounce-flash at AX, I suggest a diffuser like a LumiQuest. I would suggest a lens that is wide enough to get close shots because AX is a mad house of people. If you're into low-light there are some nice areas for evening shoots so a fast lens is nice to have.

I recommend the triangular reflectors if you plan on getting one. I bought one last year and it's been a revelation - you can hold it one-handed with a camera in the other, and if somebody else is holding it it allows them to see what they're doing, unlike a round reflector where you often block your own view while holding it two-handed. I have a Lastolite one and it's spectacular, but there are much more affordable knock-offs on Amazon.

Overall, though, I highly recommend a good on-camera speedlight with some sort of nice diffuser or bounce card. I don't really believe in on-camera softboxes, but you can get some awesome results with the indirect bounce-type ones. I like the Gary Fong ones because they go on the flash head sideways, which allows you keep them upright when the camera is vertical, which means you don't have to deal with a flash bracket.

Even in brightly-lit places like AX's lobby, a flash can be a great help since the sun is coming from overhead and the background is very bright. It lets you get the balance between the subject and background right, and if you use the flash for fill, you've suddenly got a pretty nice "studio" lighting setup with the sun coming through the glass and acting as a giant softbox main light, and then the on-camera flash as a fill light. The fill light is also critical when shooting outside under that midday July L.A. sun.